本帖最后由 ecloud 于 2015-5-25 02:32 编辑
之前拍摄的M81 & M82有核心过曝的情况,于是做了一些功课
先引用CN上面的一些见解:
The ISO for unity gain varies incredibly from camera to camera e.g. around ISO 200 for the Nikon 7000 and around ISO 4000 on the Sony A7S. It can also vary considerably within one manufacturer's range of cameras e.g. Canon. It is a function of bit depth and number of electrons for saturation.
There is an easy way to determine unity ISO for your camera, assuming it appears in the list at http://www.sensorgen.info/ and assuming the accuracy of those figures.
For a 12 bit camera the unity gain occurs at the ISO whose saturation is nearest 4096
For a 14 bit camera the unity gain occurs at the ISO whose saturation is nearest 16384
For a 16 bit camera (do they exist?) the unity gain occurs at the ISO whose saturation is nearest 65536
然后一个很有用的网站
http://www.sensorgen.info
(以上数据再次证明了Pentax K5系列的天文神机地位)
我之前对单反的ISO理解不透彻,一直根据DXO的评测数据,在K5上使用ISO1600,其实这并不是“unity gain”
按照上面的数据和原理,K5的unity gain大约在ISO250,不过此时的读出噪声偏高,比ISO1600高出将近50%
如何在Read Noise和Saturation 之间平衡,是个伤脑筋的事情
我的建议是,随机应变
如果目标光比不大,比如暗星云,例如玫瑰,可以更多向噪声方面倾斜,宽容度可以放弃一些
如果目标是星系,或者类似M42这样的大光比目标,则以宽容度为首要考虑
如果有足够的张数和充分Dithering,我个人认为Read Noise的影响不会太大
最后一点说明的是,在总曝光量一定的前提下,在后期技术过关的前提下,在一定的范围内组合张数,单张时间和ISO三个变量,最终合成后期的片子,效果几乎是一样的。唯一的区别在于信噪比和动态范围
首先单反必须要做Dithering,要Dithering起到充分效果,张数至少30张,这是底线。张数的上限以60张为宜,因为太多了提升效果就不明显了。在30-60这个张数范围内,根据目标的特点灵活分配ISO和单张曝光时间
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